Position determination is the main component of navigation systems and any location-based service. The localization of objects and persons in three dimensional spaces has become an important functionality in modern technology. After having supplied the first generation of positioning systems on satellite based systems like the global positioning system (GPS) it becomes more and more important to develop localization systems, that offer a higher resolution in urban infrastructures like street canyons. As the GPS for example requires a constant intervisibility between the user and different satellites, positioning might be difficult in areas with buildings. Furthermore the GPS-based systems may be of high costs and additional user devices are needed for localization. In times of strong enhancement of WLAN availability due to an increase of WLAN access points and implementation of WLAN functions in various electronic devices, WLAN radio waves are nearly everywhere, especially in urban areas. Additionally the demand for location-based services, that deliver services to the user with respect to his actual an updated position have been arisen recently. As known localization procedures often only base on calculating the strength of field of a propagating WLAN signal, an unavoidable source of mistakes may be buildings and other obstacles that haven't been considered during the calculation process.